The embodiments disclosed relate to systems and methods for compressing, as well as compressing and liquefying, large volumes of carbon dioxide (CO2) or gas mixtures containing carbon dioxide
Carbon dioxide is produced in a variety of industrial processes, such as combustion or decarbonization processes of fossil fuels. Carbon dioxide capture, transportation, and storage contribute to the reduction of carbon dioxide emission.
Several carbon dioxide compression or compression and liquefaction processes have been developed and used. The commonly use serially arranged compression stages and inter-cooling. A carbon dioxide stream or a stream of a mixture comprising carbon dioxide sequentially flows through compression stages arranged in series such that the pressure of the carbon dioxide or carbon dioxide mixture is gradually increased. The compressed carbon dioxide or mixture containing carbon dioxide exiting one compression stage is cooled in an inter-stage heat exchanger before entering the next compression stage, to remove heat therefrom. The stream of compressed carbon dioxide or mixture containing carbon dioxide exiting the last compression stage is eventually finally cooled, liquefied and pumped by a cryogenic pump up to the final pressure. In some cases the carbon dioxide or a mixture containing carbon dioxide is brought to a gaseous, high-density status but not liquefied.
FIG. 1 shows the carbon dioxide enthalpy-pressure diagram wherein the start (S) and the end (E) points of a compression process are illustrated. Several possibilities are available to move from point S to point E, depending upon the process used. FIG. 1 schematically illustrates three curves A, B and C representing three alternative carbon dioxide compression processes. The horizontal portions of the curves are the inter-stage cooling phases, where heat is removed from the carbon dioxide stream from one compression stage before entering the next compression stage, to at least partly remove the heat generated by the previous compression and increase gas density.
Curve A shows a process wherein inter-stage cooling and liquefaction are combined. The gas is firstly compressed along, a plurality of compression steps (5 in the example) and inter-stage cooling. The compressed gas is liquefied and finally reexpanded to a supercritical condition (point E).
Curve B shows a process where the supercritical point E is achieved by sequential compression and inter-stage cooling steps.
Curve C shows a process where the carbon dioxide or carbon dioxide-containing stream is compressed and cooled down until a condition E1 is achieved, from where the final point E is reached by pumping.
FIG. 2 illustrates a schematic of a carbon dioxide compression system according to the state of the art. Only the major components of the system are shown in the figure. A plurality of serially arranged compressor stages, labeled C1-C6, is driven by a compressor driver CD, e.g. an electric motor. Each compressor stage (six in the example shown) usually comprises a centrifugal compressor. Carbon dioxide (or a gas mixture containing carbon dioxide) is fed at IN to the first compressor stage C1 and exits said first compressor stage to enter the second compressor stage C2 and so on. In each compressor stage the carbon dioxide is subject to a compression phase to increase the pressure from an inlet pressure to an outlet pressure. Between each pair of sequentially arranged compressor stages inter-stage cooling is provided. This is schematically represented by a respective inter-stage heat exchanger (intercooler) labeled IC1, IC2, . . . IC5. The compressed carbon dioxide delivered by the downstream compression stage C6 is further cooled in a final heat exchanger IC6. Depending upon the circumstances, further processing steps can be performed, e.g. to liquefy the compressed supercritical carbon dioxide or mixture containing the same. The final pressure of the carbon dioxide at the end of the compression process shown in FIG. 2 is typically around 180-200 bar for pipeline transport or re-injection e.g. in depleted oil or gas reservoirs. Typical flow rates in carbon dioxide compression and liquefaction plants are up to 60 kg/s. This implies extremely high power consumptions to drive the compressors and the pump. Additionally, further power is required to circulate the cooling fluid (usually water) in the inter-stage cooling heat exchangers and to remove heat from the cooling fluid.